Add more drag+drop targets on the primary window

[tannerhelland]

As described by @manfromarce in #544:

[When] dropping images into PhotoDemon I have noticed a minor usability issue: if you accidentally drop a file onto the "quick start" buttons / text rather than on blank space, it seems to accept the drop but doesn't load the file. The same occurs on the left and right panels, while works properly on the canvas, images tab strip and layers panel. Do you think it would be possible to accept file drop in the whole window? Thank you in advance.